Some mobile radio standards allow the modulation type to be changed during the transmission of a data burst. These mobile radio standards include, by way of example, the “Bluetooth Enhanced Data Rate” Standard. Owing to backwards compatibility, the start of a data burst, to be precise the preamble and the access code, is always (in this Standard) modulated as in the case of the Bluetooth Normal Rate using the GFSK (Gaussian Frequency Shift Keying) modulation method. At higher protocol layers, it is possible to negotiate switching to a higher-quality modulation method after the preamble and the access code, in order to increase the data rate.
While GFSK is a modulation method with a constant envelope, the higher-quality modulation methods which are switched to after successful negotiation to switch to the higher protocol layers may be modulation methods with a variable envelope. Such higher quality modulation methods include DQPSK (Differential Quadrature Phase Shift Keying), D8PSK (Differential Eight Phase Shift Keying) and QAM (Quadrature Amplitude Modulation). Signals which have been modulated using these modulation methods must be received by a linear receiver.
To date, receivers which have been designed to receive data bursts in which the modulation method is in some circumstances switched to a linear modulation method during the transmission have been designed on a linear basis. That part of the data burst which is modulated using a nonlinear modulation method is thus also received by the linear receiver. The greater complexity of linear receivers in comparison to GFSK receivers results in a higher power consumption than a pure GFSK receiver would require for this purpose.